“At the last dim horizon, we search among ghostly errors of observations for landmarks that are scarcely more substantial. The search will continue. The urge is older than history. It is not satisfied and it will not be oppressed.” -Edwin Hubble
It still boggles my mind that a scant 100 years ago, many of the greatest astronomers and physicists of the day still thought that the Milky Way was the only galaxy in the entire Universe. It wasn’t until the 1920s that Edwin Hubble definitively showed that the great Andromeda Nebula was actually a separate galaxy from our own.
We now know that Andromeda is not only the closest large galaxy to the Milky Way, but it’s also moving towards us. In fact, it’s estimated that in the next few billion years, the Andromeda galaxy and our own Milky Way are going to merge together in a great galactic collision.
“What’s that going to look like,” you may wonder.
Well to us, inside our galaxy, other than the constellations changing slightly more quickly than they do now, and the extra bursts of star formation and supernovae that accompany major galaxy mergers, we won’t have anything all that out of the ordinary happen. The odds that even one of Andromeda’s stars, for example, will come within Pluto’s orbit of our Sun are something like one-in-ten-million.
But to someone outside our galaxy, if they had the ability to watch the collision for hundreds of millions of years, they would indeed see something spectacular.
They would witness one of the Universe’s great catastrophes: the merger of two roughly equal sized, large galaxies. Despite the fact that Andromeda is (slightly) larger than our own galaxy, we are significant enough that the great spiral structure in both galaxies is pretty much bound to be destroyed over the course of this cosmic trainwreck.
But rather than simply show you a succession of images that might represent what these two great galaxies will do, I can do you one better. Because what we’ve been able to do — with our understanding of galaxies, gas, dust, and dark matter — is to simulate in great detail what this collision is going to look like. And what we find, at many different steps along the way, is that there are interacting galaxies throughout the Universe that look an awful lot like what our simulations show. Have a look at this mind-boggling video, courtesy of New Scientist.
Of course, I wouldn’t simply leave you out there, on your own, without finding out all the information about these Hubble images of interacting galaxies, where they come from, and what they mean for us. Let’s have a look at what we just saw!
Interacting galaxy pair UGC 9618 shows two spiral galaxies at severe angles to one another in the very early stages of interaction. The two disks are still quite separate from one another, but both galaxies are extremely bright in the infrared, as the gravitational interaction between the two galaxies causes an extreme increase in star formation throughout the entire system. In fact, the star formation rate for each entire galaxy is comparable to that found in the most vigorous, active, giant star forming regions in the Milky Way.
And they haven’t even collided yet.
But later on, the two disks will begin to merge with one another, creating a sight much like that of interacting galaxies Arp 148. Shocks tens of thousands of light years in size could easily cause matter to blow outwards, perhaps even creating a brief ring-shape, as Arp 148 has. Large, galactic-sized obscuring regions that will appear as dust lanes in visible light (but that will glow brightly in the infrared) will exist in great abundance, as the star formation rate will increase above anything currently seen in our galaxy.
And then the real fun begins.
Because once that first pass-through of these two galaxies occurs, that spiral shape you know and love is ruined like a great galactic taffy-pull. Just like interacting galaxy ESO 77-14, a bridge of stars and gas will connect the galaxies, and the great dust lanes will be dead giveaways of just how distorted — in three-dimensions, not just two — these galaxies have become. Bright blue and red colors, evidence of hot young stars and ionized, star-forming regions, respectively, will both appear in abundance. While this particular pair of interacting galaxies is of slightly lower mass than the Milky Way and Andromeda, the physics largely remains unchanged.
And then, they head back towards each other once again.
The huge arms will become even more extensive, curved, and warp, and may even U-turn back in on themselves, as they do for merging galaxy pair VV 705. Shown roughly mid-way through a merger, these flailing arms stretch more than twice the length of either original galaxy.
But despite the distortion and the chaos of the merging process, at the end of the day, gravity will have the final say.
As ESO 148-2 begins to show, the two cores will eventually merge together into one new core, the vast majority of the matter strewn about in the collision will settle down and recollapse back onto the new, even more massive object, and over time, this will likely settle into a giant elliptical galaxy.
And so, you might ask, where did all these great Hubble images of interacting galaxies come from? And where can you get more of them?
Three years ago, back when Hubble turned 18 (!), it took 59 deep, high-resolution images of interacting galaxies, which are all freely available to download, thanks to the Hubble Heritage team!
And so even though we — and all life on Earth — won’t be around to see it, we can still know what the future of our Milky Way holds. Even when, well, it’s going to be less than 50% Milky Way! And that’s what our future will look like to an outsider.